The first ever, large diameter slurry TBM application in India has proven its worth in the city of Bangalore.

Working through the undulating interface between fresh granite bedrock and the weathered granite layers beneath soft wet residual soils at the surface, the last of the pair of 6.5m diameter Hitachi Zosen slurry TBMs to finish broke through on Monday, 17 March. After an initial outbound drive of 221m, it took a U-turn and headed back under 23 sets of tracks at the main railway station at Bangalore to finalise 5.1km of hard won running tunnel excavation for the first section of the city's underground metro alignment.

BMRCL Managing Director Kharola (second from left) and Chief Engineer N. P. Sharma (third from right) commended Chris Reeves (first left) andmembers of the CEC-Soma-CICI JV for their work

At the breakthrough ceremony, the work was commended by Pradeep Singh Kharola, Managing Director of the BMRC, Bangalore Metro Rail Corporation Ltd, and its Chief Engineer for the design and underground works, N. P. Sharma. Both senior managers of the client organization acknowledged the very difficult conditions under which the tunneling has been completed by contractor CEC of Taiwan, with its Indian subsidiary CICI completing the ramps and cut-and-cover sections, and JV partner Soma managing open box excavation of the contract's four underground stations.

"The ground conditions and the logistics here in the centre of Bangalore are very demanding," admitted BMRCL Managing Director Kharola, on the occasion of the breakthrough. "CEC has done an excellent job to complete the UG2 contract's running tunnels to a high standard and quality."

"We specified closed face TBM technology for the tunnelling," said Chief Engineer Sharma, "to control ground excavation and surface settlement, but left the selection of the actual system to the contractor's preference. The slurry technology used by the CEC team has worked very well in these extreme mixed face geological conditions at the tunnel horizon."

Contract UG2 is the east-west line of the Bangalore Metro

The CEC-Soma-CICI JV was awarded the UG2 contract in March 2010 for a contract price of 10 billion Rupee or about US$216 million. To complete the total 4.8km x 5.6m i.d. running tunnels CEC selected the slurry technique and procured two 6.5m diameter slurry TBMs from Hitachi Zosen of Japan with separation plants for each also from Japan.

The complex geology at tunnel alignment comprises a mix of hard massive granite, at more than 300MPa UCS and few joints or fissures, in the bottom of the face; layers of weathered granite in the centre, with many fresh granite blocks and boulders in the weathered matrix; and residual soils of wet running sands, clays and gravels at the top.

"Rather than presenting in uniform layers," said Kumar Kanchan, Geotechnical Manager for the JV, "the interfaces are undulating, and a tunnel face is almost always consisting of two of these geological types if not all three at once."

Mixed conditions from hard rock to wet sands

"The mixed conditions were very difficult for TBM operations," explained Chris Reeves, Project Leader for the JV. "Fine pressure control of the slurry in the excavation chamber was required to prevent a raveling situation in the wet residual soils at the top of the excavation chamber, while dealing with a weathered granite matrix and boulders in the bottom or - most damagingly for the cutting tools - running into massive fresh hard rock in the invert. The slurry closed faced system offered the best solution for maintaining fullface control and avoiding surface settlement. Our TBM operators from Thailand, with local labourers and under the leadership of Tunnel Manager Wichien Conjaisue, were able to maintain tight control of the slurry excavation process."

The TBMs operated at a relatively shallow depth of one to two machine diameters below the surface and at an operating pressure of about 0.2-0.7 bar, always less than 1 bar, with only the upper layers of residual soils and the weathered granite matrix requiring pressurised slurry face support. At all times, muck haulage was via the slurry transportation circuit to the separation plant and through a stone crusher located on the outbound slurry line in the shield and outside the excavation chamber.

Track support work ahead of TBM underpass

During TBM advance, heavy wear of the tools was caused by the highly abrasive 55-60% quartz content of the granite and by the breaking of the cutter discs, hubs and bearings as they crashed into the interface of softer material and the very hard bedrock. Interventions for frequent tool changes were under compressed air or in free air in safe havens of grout injection blocks installed from the surface.

Behind the TBMs, the mechanical erectors built the 1.5m wide rings of rebar reinforced universal trapezoidal segments connected with spear bolts on the longitudinal joints and dowels are at the circle joint. The composite EPDM/hydrophilic gaskets and the clean build of the rings, without lips or steps and within 20 minutes by the experienced crews, made for a high quality finish to the segmentally lined tunnels.

Over the course of the twin 5.1km long running tunnel drives, a major challenge was the scheduling of the tunnel boring with the station excavations. At each of the four station boxes, the machines had to be lifted after breakthrough and moved to the far end of the box excavations for relaunch. "We had three launches and three retrievals for both machines," explained Mateo Pagsisihan, Planning Manager, "and for the two final drives to cut-and-cover transition zones to the surface and elevated alignments of the east-west line, we used one TBM and completed U-turns at the end of the first drive to complete the parallel tube, running the slurry and service lines around the corner and into the return drive. For all slurry drives we were able to leave the treatment plants in the original set up location, with the slurry circuit extending for up to about 3km at the greatest distance."

Slurry TBM breakthrough in a full face of hard rock

Adding to the geological and logistical challenges were the environmental issues of working in the heart of a major city and amid the extreme traffic and urban congestion of a major city in India. This was particularly so on the final U-turn drive of the twin tunnels that passed twice under the 23 sets of tracks and platforms at the Bangalore Central Railway Station. Many of these tracks are for passenger train sidings and freight train stunting, while others were the tracks of vital commuter rail lines to the suburbs or intercity links with Delhi and Chennai and other major city connections.

Amid the urban congestion, there was little opportunity to sink investigation boreholes and the owners and operators of the station and its tracks were reluctant to allow any access to their property or possible disruption to their train services. This did cause the one significant TBM boring incident on the contract.

While passing under the tracks on the first leg of the last drive, the TBM hit a pocket of very wet fine running sand that ravelled fast. The TBM kept moving and it was after setting two or more rings that a sinkhole appeared on the surface above the shield, behind the cutterhead and in front of the articulation joint. This was quickly filled and stabilized and the railway company was pleased to have the CEC tunnelling teams on the property, taking site investigation boreholes, pretreating the ground with consolidation grouting and injecting from the surface secondary grouting around the segmentally lined tunnel to confirm the quality of the geology and ensure no repeat of the incident. The rail operator also agreed to installation of lateral support beams under the tracks where they passed across the tunnel alignment, to ensure no disruption of train services during the drives.

Quality finish of the segmentally lined tunnel

Slurry TBM technology was not the easiest method of excavation in the mixed ground conditions nor the cheapest admitted Bob Moncrieff, TBM Consultant and adviser to CEC, but it was the best solution. "It would have been very difficult to maintain the necessary face pressure using an EPBM," he said. "The Hitachi machines were selected for several reasons including their robust design, installed power, efficient slurry circuit pumping and running costs and the location of the crusher outside the excavation chamber. They have performed well in these very particular ground conditions."

Work on the 174-week contract started in March 2010 and was due for completion by September 2013. Due to various constraints however, including delayed access to site, an extension of time has been granted to complete by December 2014. Tough TBM tunnelling conditions also slowed top advance rates of eight to 10 rings per day in full faces of residual soils or soils and weathered rock materials, down to two or three rings a day when grinding through faces of hard, dry massive rock. The slurry system was vital in both situations, providing a cooling effect on the TBM when working at full power as well as assisting to reduce wear through the lubricating effect of the polymer based slurry. With the last drive of the last TBM now completed work on the stations and open cut works are on track to meet the December 2014 end date.

Following that the two Hitachi TBMs will be looking for new applications and this may come for CEC and its JV partners on Phase 2 of the Bangalore Metro which includes 14.5km of twin tube underground alignment and which has government approval for a possible award of underground contracts before the year end 2014. Elsewhere in India, CEC and its CICI subsidiary is working with two Robbins EPBMs on an underground alignment contract for the Jaipur Metro and in Delhi on Phase III of the Metro using two Herrenknecht EPBMs. Neither contracts or teams are experiencing the difficult conditions mastered during the battles with bedrock in Bangalore.

A full site visit report by TunnelTalk to the Bangalore Metro works will publish in the coming weeks.

References

Underground advance for Bangalore Metro - TunnelTalk, July 2010
Bangalore Metro slurry TBMs inaugurated - TunnelTalk, May 2011

Conference reference

WTC2012, Bangkok, Thailand: R. L. Moncrieff - The first large diameter slurry TBM's in India - Selection of the tunnelling system for the Bangalore Metro

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